Semiconductor lasers typically are fabricated on a wafer by growing an appropriate layered semiconductor material on a substrate through Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) to form an epitaxy structure having an active layer parallel to the substrate surface. The wafer is then processed with a variety of semiconductor processing tools to produce a laser optical cavity incorporating the active layer and incorporating metallic contacts attached to the semiconductor material. Laser facets typically are formed at the ends of the laser cavity by cleaving the semiconductor material along its crystalline structure to define edges, or ends, of the laser optical cavity so that when a bias voltage is applied across the contacts, the resulting current flow through the active layer causes photons to be emitted out of the faceted edges of the active layer in a direction perpendicular to the current flow. Since the semiconductor material is cleaved to form the laser facets, the locations and orientations of the facets are limited; furthermore, once the wafer has been cleaved, it typically is in small pieces so that conventional lithographical techniques cannot readily be used to further process the lasers.
The foregoing and other difficulties resulting from the use of cleaved facets led to the development of a process for forming the mirror facets of semiconductor lasers through etching. This process, as described in U.S. Pat. No. 4,851,368, the disclosure of which is hereby incorporated herein by reference, also allows lasers to be monolithically integrated with other photonic devices on the same substrate. This work was further extended and a ridge laser process based on etched facets was disclosed in the IEEE Journal of Quantum Electronics, volume 28, No. 5, pages 1227-1231, May 1992, and U.S. application Ser. No. 11/356,203 the disclosures of which are hereby incorporated herein by reference.
Distributed feedback (DFB) lasers use a distributed diffraction grating to generate a single wavelength output. The relative position of the facets and the grating are of critical importance in the performance of these lasers, as discussed by Streifer, et al. in a paper entitled “Effect of External Reflectors on Longitudinal Modes of Distributed Feedback Lasers,” IEEE Journal of Quantum Electronics, Volume QE-11, pages 154 to 161, April 1975. Unfortunately, cleaved facets can only be placed within a desired position only to a few microns causing random phase variation between the facets and the grating, and are therefore unpredictable in laser performance and characteristics. As such, these lasers have a limited yield by design.
The key performance characteristics for DFB lasers are whether the device operates in a single longitudinal mode (SLM) and whether it has a high side mode suppression ratio (SMSR). The impact of this phase variation on yield is discussed by Kinoshita, et al. in a paper entitled: “Yield analysis of SLM DFB lasers with an axially-flattened internal field,” IEEE Journal of Quantum Electronics, Volume QE-25, pages 1324 to 1332, June 1989, and David, et al. in a paper entitled: “Gain-coupled DFB lasers versus index-coupled and phase shifted DFB lasers: a comparison based on spatial hole burning corrected yield,” IEEE Journal of Quantum Electronics, Volume QE-27, pages 1714 to 1723, June 1991.
DFB lasers can be processed with photolithographically defined etched facets, but the alignment accuracy of the photolithographic system together with the angular misalignment between the placement of the grating and the etched facet is insufficient to deterministically specify the phase between the facet and the grating across a wafer. Since high yield DFB lasers are extremely desirable, an improved structure and method for making etched-facet semiconductor DFB lasers are needed which address the foregoing issues.